This invention relates to stencil screen printing machines.
Stencil screen printing machines have been known for decades, typically being of the sheet stock print type, or the web stock print type. The machine employs a reversible carriage which supports the stencil screen frame and screen, a rotational print cylinder having sheet grippers, and a squeegee for moving ink through the screen onto a substrate sheet or web fed over the cylinder. The common way of feeding sheet stock is to grip the front edge via grippers positioned in an axial trough of the cylinder. The squeegee is lowered to push the screen down to the stock on the print cylinder. The cylinder and stencil screen carriage are operably connected to the drive mechanism to thereby be driven in synchronism. The cylinder rotates and the screen is linearly advanced with the sheet stock, pulling the sheet that is between the cylinder and the screen. After each sheet is printed, it is stripped from the cylinder by stripper elements, the squeegee is elevated from the cylinder and the screen, the cylinder rotates back to the starting position, and the screen frame and screen are reversed to the starting position to accommodate the next piece of stock.
Keeping the carriage drive, print cylinder, and stripper elements in synchronism during the drive condition is essential, but can be troublesome and requires complex mechanism in the press. Also, although the known presses are capable of printing sheet stock of different lengths and are capable of printing print patterns of different lengths, the screen frame carriage must shift the entire length of the maximum length sheet capable of being printed, since the linearly driven carriage must stay in synchronism with the rotationally driven print cylinder. This results in lost time and extra equipment wear when printing shorter stock and/or shorter print patterns.
Another troublesome factor with screen printing equipment is the distortion of the print pattern which can occur as a result of the squeegee necessarily pressing and bowing the stencil screen down to the underlying print cylinder. This lowers the print quality and can also cause excess scrap product, especially when the specifications of the printed product are highly exacting.
The novel stencil screen printing press herein simplifies the printing operation and the equipment. The screen frame carriage is driven independently of the rotational print cylinder. The carriage drive is controllable to reciprocate a variety of selected distance amounts, thereby readily accommodating different length stock and/or print patterns, without concern for keeping the carriage drive in synchronism with the print cylinder drive. A servo motor drives the carriage forwardly and rearwardly the selected amount to suit the length of the stock and print pattern. The cylinder is not positively driven, but rather is free wheeling in nature. It has no sheet grippers. The sheet stock is advanced and the cylinder is rotated in synchronism with the sheet stock by the advancing screen as gripped by the squeegee against the sheet stock and cylinder.
The novel apparatus does not distort the stencil screen by the lowered engaging squeegee. The cylinder surface and the squeegee are both in the same plane as the infeeding stencil screen and print stock. The cylinder is elevated to this screen plane when the squeegee is lowered to this screen plane. Hence, the screen is not bowed (i.e., distorted) by the squeegee. Therefore, the print pattern is not distorted, and this results in higher quality product and less potential scrap. This feature could also be used for printing web stock.
The print cylinder employs vacuum at its peripheral portion engaging the sheet stock leading edge, to pull the stock and infeed it as the cylinder rotates. The cylinder, however, has positive pressure (i.e., blowing at its peripheral portion engaging the part of the sheet stock already printed) to separate the printed stock from the cylinder surface.
These and other features, objects, and advantages of the invention will become apparent from studying the following specification in conjunction with the drawings.